In the field of coal pulverizing mills, there are generally two types of mills characterized by the manner in which the pulverized coal is delivered from the mills to a combustion chamber: "suction" mills using exhauster fans to pull the pulverized coal fines from the mill through discharge pipes; and, "pressurized" mills which are fanless and typically entrain the pulverized coal fines in a stream of pressurized air originating at the mill itself.
Each type of mill presents its own problems with respect to the goal of supplying an even, balanced flow of coal fines through multiple pipes to multiple burners in the combustion chamber. In suction mills, for example, the exhauster fan itself tends to throw coal in an unbalanced stream, with heavier particles settling out to one side of the flow through the pipe and lighter fines on the other. In pressurized mills without exhauster fans, distribution problems tend to occur as a result of the varying lengths of discharge pipe leading from the top of the classifier to the various burners around the combustion chamber. Shorter lengths of discharge pipe generally run rich, while longer lengths of pipe tend to run lean. This rich/lean imbalance among the various burners in the combustion chamber produces the usual problems: loss on ignition (LOI) contamination of the ash byproduct; NOX formation; fireball distortion and waterwall erosion; and others known to those skilled in the art.
One common technique for trying to balance coal flow in pipes of different length is known as "clean air flow testing", in which orifice plate restricters are placed in the shorter pipes to try to balance air flow with respect to the longer (slower, lower volume) pipes in an air-only test procedure. The problem with clean air flow testing is that, having balanced air flow in a theoretical test, the introduction of coal fines produces fundamentally different results than the air-only testing would indicate, and the orifice plates worsen distribution problems among and within the pipes.
Dynamic classifiers power-rotate an array of vanes in the classifier cone to decelerate larger particles of coal and encourage lighter fines to travel up and out the classifier into the discharge pipes. It has been found, however, that the use of dynamic classifiers still results in + or -20% differences in distribution among the pipes (resulting in a 40% variance).